The present invention relates to a disk drive, a positioning method for a head, and a servo system. More particularly, the invention relates to a disk drive for compensating for repetition errors by means of a filtering process, and an associated head-positioning method and servo system.
The devices that use various forms of media such as optical disks and magnetic tapes, are known as data storage devices. Among these devices, hard-disk drives (HDDs) have come into widespread use as storage devices in computers, and are one type of storage devices indispensable in current computer systems. In addition, HDDs are not only used in computers; the excellent characteristics of HDDs are increasingly expanding their applications such as the removable memories used in dynamic image recording/reproducing apparatus, car navigation systems, digital cameras, or other products.
The magnetic disks used in HDDs each have multiple tracks formed into the shape of a concentric circle, and address information (servo information) and user data are stored onto each track. A magnetic head formed of a thin-film element can read or write data by accessing a desired region (address) in accordance with the address information. The magnetic head is fixed to a slider, and the slider is further fixed to a carriage capable of oscillating. The carriage is oscillated by a voice coil motor (VCM), and thus the magnetic head can move to a desired position on the magnetic disk. The VCM is driven by a VCM driver, and the VCM driver drives the VCM by supplying an electric current thereto according to the control data sent from a controller.
As mentioned above, each track has a data region into which data is stored, and a servo region into which servo signals are stored. A track ID, servo sector IDs, burst patterns, and others are stored as servo data into the servo region. The track ID and the servo sector IDs identify the addresses of the track and the servo sectors, respectively. The burst patterns contain information on the relative position of the magnetic head with respect to the track, and are used during track following. The burst patterns are each an array of regions in which signals were stored radially onto the disk at fixed intervals, and one burst pattern is constituted by multiple banks of signal storage regions different from one another in terms of phase.
Data reading from or writing onto the magnetic disk is executed while the position of the magnetic head is being confirmed by means of servo signals in a rotating condition of the magnetic disk. The servo signals that have been read by the magnetic head are computed by the controller. The value of the electric current to be supplied to the VCM is determined from the relationship between the current position of the magnetic head and the desired position thereof. The controller creates a control signal DACOUT for indicating the calculated electric current value, and supplies the current to the VCM driver. In case of a deviation, the carriage is driven to compensate for the deviation and the position of the magnetic head is controlled.
The servo signals, although usually recorded on the magnetic disk by use of a servo track writer, are not always recorded in perfect round form since vibration or the like can occur during recording. The particular error appears as repeatable run-out (RRO) during track following. If the RRO is significant, a track following error can result since the magnetic head (servo system) cannot follow the RRO. When the frequency components of the repeatable run-out error are limited, for example, if the run-out is great only at the rotating frequency components of the disk, it is known that the run-out can be compensated for by inserting a filter with a peak at that frequency into the servo system (for this method, refer to Patent Document 1 (Japanese Patent Laid-Open No. Hei 08-328664), for example).
It is generally known that repeatable run-out can be compensated for by integrating the state variables that have been input in the past. Since the filter used in this method has multiple peaks associated with the frequency of the repeatable run-out error, the filter can remove all of the repeatable run-out error components.
Patent Document 2 (Japanese Translation of PCT for Patent Application No. 2002-544639) discloses a technology for removing RRO components from a position error signal (PES), the deviation between a head position signal and a target signal. Removal of the RRO components from signal PES makes the magnetic head follow a substantially round path, not the shape of the track. Since the servo system operates independently of the RRO, it is possible to prevent the occurrence of a track following error due to the RRO.